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Earth Materials Unit: Igneous Rocks
and Processes
Text: Chapters 3 and 4
Lab: Laboratory Five
Name________________________________
Geology 12: Igneous Rocks
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Igneous Rocks and Processes
Purpose:
To classify igneous rocks
and features by their physical and
chemical properties.
-By the end of this unit, students are
expected to be able to:
1. Describe factors affecting crystal size:
-cooling rate, flow behavior, viscosity
(nucleation)
2. Relate texture to rate of the
crystallization for intrusive (plutonic) and extrusive (volcanic) igneous rocks:
3. Identify and classify igneous rocks according to:
Texture
Coarse Grained
Fine Grained
Vesicular
Glassy
Fragmental (Pyroclastic)
Composition
Felsic
Intermediate
Mafic
Ultra-Mafic
4. Describe the features of the following igneous rocks: (Text pg. 75-81, Lab pg. 92, 93, 97-103)
Granite
Tuff
Pegmatite
Diorite
Rhyolite
Pumice
Gabbro
Basalt
Porphyry
Peridotite
Volcanic breccia
Andesite
Obsidian
5. Explain the order of crystallization of minerals from magma (Bowen’s Reaction Series). (Text
pg. 65, Lab pg. 96)
6. Distinguish among these volcanic features: (Text pg. 85; 98-110, Lab pg. 114-116)
Shield volcanoes
Fissure eruptions
Lava plateaus
Composite or strato volcanoes
Columnar jointing
Cinder cones
Volcanic domes
8. Distinguish among the following volcanic features by composition, flow behaviour and their
resulting rock or feature: (Text pg. 93-98)
Lava
Aa
Ash flows or nuee ardente
Pahoehoe
Geology 12: Igneous Rocks
Pillow lava
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9. Identify and describe: (Text pg. 81-86, Lab pg. 114-116)
Batholiths
Stocks
Sills
Plutons
Geology 12: Igneous Rocks
Dikes
Xenoliths
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Geology 12: Igneous Rocks and Processes
Part A: Magma
-Magma is the parent material of all igneous rocks and forms
through the partial melting of Earth’s crust and upper mantle.
-The nature of the parent magma determines the composition
of the igneous rock it produces. As such, it is important to
understand how different types of magma are created!
Magma Formation
-Magma forms through the melting of solid rock in Earth’s
crust and upper mantle due to:
1. Heat:
-As we descend deeper into Earth’s interior temperatures
increase by an average of 20 to 30OC per km in the upper
crust. This is called the Geothermal Gradient.
-By itself, the geothermal gradient does not generate enough
heat to melt rocks in the lower crust and upper mantle. How
else could we generate the extra heat needed?
a) _______________________________________________
_________________________________________________
b) ____________________________________________________________________________
______________________________________________________________________________
c) ____________________________________________________________________________
______________________________________________________________________________
2. Pressure:
-Pressure also increases with depth and raises the melting points
of rocks. How can we reduce pressure to lower a rock’s melting
point?
a) _________________________________________________
___________________________________________________
______________________________________________________________________________
Geology 12: Igneous Rocks
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3. Volatiles
-Water and other volatiles lower the melting point of rock and
their effect is magnified at depth.
Magma Evolution
-Magma can be altered to become the parent of a variety of
rock types through several processes:
1. Bowen’s Reaction Series
-Minerals crystallize according to their melting points
(mafic to felsic).
2. Fractional Crystallization
-As such, the composition of magma can be changed by
the removal of denser, early-formed mafic minerals by
crystal settling. The remaining magma is then more felsic.
3. Partial Melting
-Bowen’s Reaction Series is reversed when rocks are heated. This
means that felsic minerals will melt first as they have a lower
melting point. The incomplete melting of rocks can then produce
different magma compositions.
4. Assimilation
-Magma can melt and incorporate surrounding country rock,
changing the magma’s composition.
-For example, mafic magma can assimilate felsic continental crust
to produce an intermediate magma (and eventually rock).
5. Magma Mixing
-Occurs when one magma body intrudes on, and mixes with,
another to produce an intermediate composition.
Geology 12: Igneous Rocks
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Magma Formation and Plate Tectonics
-Igneous activity (magma and rock formation) occurs primarily at or near tectonic plate boundaries.
1. Mafic Magma and Rocks
-Form at Divergent Plate Boundaries where low overhead pressure allows decompressional melting of the
asthenosphere.
2. Intermediate Magma and Rocks
- Form at Convergent Plate Boundaries where partial melting of the subducting asthenosphere produces
basaltic magma that evolves into intermediate magma through differentiation, assimilation and magma
mixing.
3. Felsic Magma and Rocks
- Form at Convergent Plate Boundaries where hot mafic magmas produced near a subducting slab causes
the partial melting and assimilation of continental (granitic) crust.
*Some igneous rocks form within plates (and not at their boundaries) via mantle plumes creating
localized hotspots or volcanoes as they rise through oceanic or continental crust.
Geology 12: Igneous Rocks
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Part B: Igneous Rock ID
Igneous Rock Composition (Text 68-70,
Lab 106-107)
-Felsic: ____________________________
__________________________________
__________________________________
-Intermediate: ______________________
__________________________________
__________________________________
-Mafic: ____________________________
______________________________________________________________________________
-Ultramafic: ____________________________________________________________________
______________________________________________________________________________
Igneous Rock Textures (Text 70-75; Lab 104-105)
-Texture is used to describe the appearance of a rock based on its size, shape and arrangement of
interlocking mineral crystals. It is important as it allows inferences to be made about the rock’s formation
environment.
-Three factors affect igneous rock textures:
a) Rate of Cooling: ______________________________________________________________
______________________________________________________________________________
b) Amount of Silica Present: _______________________________________________________
______________________________________________________________________________
c) Amount of Volatiles Present: ____________________________________________________
______________________________________________________________________________
Geology 12: Igneous Rocks
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-Types of igneous rock textures include:
a) Aphanitic: ___________________________________________________________________
______________________________________________________________________________
b) Vesicles: ____________________________________________________________________
______________________________________________________________________________
c) Glassy: _____________________________________________________________________
______________________________________________________________________________
d) Phaneritic: ___________________________________________________________________
______________________________________________________________________________
e) Porphyritic: __________________________________________________________________
______________________________________________________________________________
f) Pegmatic: ____________________________________________________________________
______________________________________________________________________________
g) Pyroclastic: __________________________________________________________________
______________________________________________________________________________
Geology 12: Igneous Rocks
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Part C: Intrusive Igneous Rocks
-Most magma never reaches Earth’s surface, but
instead crystallizes into intrusive igneous rock
bodies.
-Use your text (81-86) and lab book (114-116) to
define the following terms:
- Country Rock: ___________________________
________________________________________
-Discordant: ______________________________
___________________________________________________________
-Concordant: ________________________________________________
___________________________________________________________
-Dike: _________________________________________________
_______________________________________________________
-Sill: _________________________________________________
_____________________________________________________
*Determine the type of boundaries and igneous intrusion depicted in the diagrams below:
Intrusive Boundary: ____________________
Intrusive Boundary: ____________________
Intrusion: ____________________
Intrusion: ____________________
Geology 12: Igneous Rocks
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-Laccolith: _______________________________________ 1. 75 mya magma injected under overlying sedimentary layers
________________________________________________
-Batholith: _______________________________________
2. Weathering and erosion remove overlying layers and expose
________________________________________________ laccolith (resistant to weathering as igneous rock)
-Stock: __________________________________________
________________________________________________
-Volcanic Pipe: ___________________________________
________________________________________________
-Volcanic Neck: ___________________________________
3. Crown Butte, Montana is an eroded laccolith,
_________________________________________________
Part D: Volcanoes
Magma Composition and Viscosity (Text 91-93)
-The nature of a volcanic eruption and even the shape of a volcano
is determined by the type of magma that feeds it. In particular,
the viscosity of the magma is extremely important.
-Viscosity: ____________________________________________
______________________________________________________
a) Composition: _________________________________________________________________
b) Temperature: _________________________________________________________________
c) Dissolved gases: ______________________________________________________________
Ejected Materials (Text 93-98)
-Depending on the magma type, several different types of lava flow are possible:
a) Pahoehoe: _______________________________________________
__________________________________________________________
__________________________________________________________
Link: Hot Pahoehoe Lava on Kilauea Hawaii
Geology 12: Igneous Rocks
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b) Aa: __________________________________________________
________________________________________________________
________________________________________________________
Link: Kilauea a’a Lava Flow
c) Pillow Lava: _________________________________________________
______________________________________________________________
d) Ash: _______________________________________________________
______________________________________________________________
Link: 2010 Iceland Ash
Cloud (It continued to
spread across the European
Continent in the following
days)
e) Nuee Ardente: ___________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
f) Lahar: _________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Geology 12: Igneous Rocks
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Volcano Types (Text 98-110)
Shield
Cinder Cone
Composite Cone
Lava Dome
Shape
Lava Type
Other
Characteristics
Dangers
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Geology 12: Igneous Rocks
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